Planar machines and saw cutting machines for road maintenance work generally drive the cutter heads and saws in a rather conventional manner directly from a power unit or through a hydraulic pump and motor. In U.S. Pat. No. 2,311,891, a road groove cutting tool is directly driven from a separate engine provided for such purpose, through a transmission mechanism and the traction units are driven through a slip-clutch from another engine. Thus, if the cutting tool is held back by road conditions, the clutch will slip to arrest the advance travel of the machine so as to reduce the load on the cutting tool engine. U.S. Pat. No. 2,817,275 discloses a planar machine in which the cutter head is chain and sprocket driven from an engine through a clutch mechanism. The machine is manually propelled so that advancement of the cutter is readily controlled relative to cutting conditions. In the roadway slotting machine of U.S. Pat. No. 3,321,250, the cutter is belt driven from an engine which is also used to drive traction wheels through means including a hydraulic pump to motor to differential arrangement. The road groove cutter in U.S. Pat. No. 3,333,897 is in a direct engine drive connection while in U.S. Pat. No. 4,139,318, the cutter head and traction units are driven in a conventional manner.
In the prior art devices having a belt driven cutter or a cutter having a slip-clutch connection with the power unit, the cutter rotational speed and inertia force are appreciably decreased or stopped when an obstruction is encountered. The cutting operation is thus either stalled or takes place at a slow speed with resultant loss of time and increased expense. Where the advance travel of the machine is continued concurrently with a positive drive of the cutter head as in U.S. Pat. No. 4,139,318, for example, the cutter head is susceptible not only to heavy tooth damage, but also damage to the power unit and/or its drive system.